Traditionally, ultraviolet (UV), visible and infrared electromagnetic or light energy is collected using semiconductors with bandgap energies tuned to the desired photon energy to be collected. Alternatively, light energy may be converted into thermal energy by an absorber and then the heat energy may be collected by traditional thermal energy collectors, such as sterling engines, steam engines or other methods. These major solar energy collection technologies may be further grouped as follows: 1) inorganic, semiconductor based photovoltaic (“PV”) generation, 2) organic based PV generation, 3) nanotechnology, which includes carbon nanotubes and quantum dots, and 4) solar thermal or solar concentrator technologies. Silicon (semiconductor-based) PV technologies, solar thermal technologies and solar concentrator technologies are the most widely used currently commercially available and mature technologies.
In some cases, photovoltaic technologies use discrete bandgap potentials generated by p-doped and n-doped semiconductor material to collect energy from light. Typical inorganic PV efficiencies may range from 10% for the single junction cells up to around 28% for triple junction PV cells. PV technology is limited physically to less than 33% energy collection efficiency by bandgap energy collection limitations and by semiconductor electrical resistance.
Additionally semiconductor-based PV has high costs associated with the materials used and the manufacturing process. The material costs include the high cost to produce pure wafers and the use of rare and expensive materials. The manufacturing costs include the huge capital cost to build a semiconductor facility, the control of toxic materials used and the cleanliness requirement to prevent any impurities from doping the product while under manufacture. These costs may be reasonable for integrated circuit (IC) electronics because most, if not all features required to permit the IC to function may be located in a small area, with many devices being produced on a single wafer.
In contrast to the needs for IC electronics, solar collection technology requires large surface areas to collect light. The large area requirement provides an inherent limitation to devices that use expensive processes because of the surface area cost to generate PV-based solar energy. Therefore, there are cost restrictions for the use of inorganic PVs for many energy markets.
Another major downside for current PV technology is the use of toxic materials during processing of PV devices and in the final PV product. After the end of life of current PV devices, the environmentally toxic or hazardous materials contained in such devices creates an environmental disposal problem.
Another category of technology that may be used to collect photon energy includes sensors that use voltage enhanced field emission. Such devices use high voltages to detect typically low-intensity photons using the photoelectric effect. Such devices have a net energy loss and amplify a signal using an external power source. These devices consume more power than they produce and are not useful as energy collectors.
Another category of technology is based on a recent finding that electric field enhancement on existing detectors may improve the performance of photo-detectors. This method of enhancement uses surface structures to enhance the electric field in desired locations. The enhanced electric fields created in accordance with this category of technology allow greater electron mobility in devices.
Although conventional antennas convert electrical current from (to) antenna structures to (from) far-field, optical antennas may also be used for near-field applications such as imaging and touchscreen displays by contact sensors. Antenna structures may be designed using rigid or flexible substrate, metallic, and dielectric layers to give more integration flexibility and enable electromagnetic field manipulation through leveraging the geometrical shape of the optical antenna arrays at the macro-scale. Such combination of micro-geometrical structure at the unit cell level and macro-geometrical feature at the array level provide more degrees of freedom in defining the virtual values of the effective permittivity and permeability of the array. For instance, using inner layers of metamaterial structures with dispersive properties within the light spectrum to improve the optical antenna efficiencies and enable manipulation of the electromagnetic absorption and refraction at the air and inner interfaces. Furthermore, optical antennas may perform such conversion with and without 1) thermal conversion, 2) using plasmon frequencies of metal, or 3) leveraging quantum properties of material used to build such structures.
As promising as these new technologies are, most are restricted to collecting light using discrete quantum energy bands, which imposes the same inherent efficiency limitations as semiconductor PV technologies. Increasing the number of junctions or wells increases the number of bandgaps and increases the useable energy, which results in increased efficiency across the visible light spectrum. As with inorganic semiconductors, this approach has a downside because each new well or junction creates a layer that may interfere with (mask) the well or junction below it and increase the path length of both the light and the free charge, which increases the losses from absorption and electrical resistance. Furthermore, nanotechnology and quantum dots still have issues with toxicity, with the ability to manufacture and with efficiency.
The underlying structures of these optical antennas may be manufactured more economically and with high-yield allowing these small and simple structures to be used in various applications where size, cost, efficiency or precision is relevant.